The invention relates to protective low oil pressure sensor switches for turning off associated machinery to protect same in the event of low oil pressure. The invention particularly arose during development efforts to detect a condition of low oil pressure across the lubrication pump of a refrigeration compressor, though the invention is not limited thereto.
Lack of proper lubrication is a dominant cause of refrigeration compressor failures. This can be caused by a shortage of oil in the system, logging of oil in the evaporator due to insufficient refrigerant velocities, shortage of refrigerant, refrigerant migration or floodback to the compressor crankcase, failure of the oil pump, or improper operation of the refrigerant control devices. To prevent such failures, oil pressure safety controls have been used.
Prior oil pressure safety controls typically are provided by a tee fitting on the oil pump pressure port with a Schrader valve in the branch connection. A high pressure capilliary connection is provided to the tee run connection by long copper tubing typically wound in a coil. The Schrader valve allows a service man to check the oil pump outlet pressure while the system is in operation without disturbing the oil pressure safety control connections. The low pressure side of the pump is sensed via the crankcase through a connection provided by a long copper tube which is also coiled. An oil pressure control operates on the difference between the oil pump outlet pressure (high pressure side of the pump) and the crankcase pressure (low pressure side of the pump) which is the net or differential oil pump pressure. Mechanical contacts typically provide a switching function in response to a given pressure differential. Current relays and the like may provide delays as necessary to prevent false tripping or afford hysteresis as desired. The long coiled copper tubing provides a damping function for damping high pressure pulses and transients from the high pressure side of the pump, to prevent false triggering.
As an example, a Copeland Part No. 085-0062-00 refrigeration compressor may use a Penn Model No. P45NCA-12 or a Ranco Model No. P30-5826 or a Robertshaw Model No. PD21-2502 oil pressure safety control, as noted in Copeland Application Engineering Bulletin AE-1095-R9, Sept. 1, 1981, which also notes other part numbers and model numbers.
The present invention addresses the particular problems to be overcome in a low oil pressure sensor directly mounted on the pump body and directly inserted into and across the high and low pressure sides of the pump to sense differential pump pressure. The violent pressure fluctuations and high pressure surge pulses must be withstood without false triggering. Furthermore, since the sensor is exposed to the harsh internal pump environment, reliability is a significant consideration, especially longevity and possible contamination of prior mechanical contacts. Furthermore, an extremely compact design is required because of the space limitations of direct internal pump sensing.
A variety of engineering problems were thus presented. The sensor must have a durability that outlives the compressor system. The sensor must be contained in a very small package. The sensor must live in an environment that is hostile to numerous materials. The sensor must be insensitive to dynamic pressure transients and respond only to mean pressure conditions while still maintaining a low level hysteresis.
In an initial effort, a simple mechanical switch was tried and failed. The fluid medium, differential measurement, vibration and dynamic pressure conditions virtually eliminated the use of any type of sensor that uses a mechanical contact or variable resistor.
The desired life span and fluid media eliminated the use of a diaphragm with resistive strain gauges. Aging of the bonding and track breakage could cause considerable field problems.
The small package size prohibits the use of signal conditioning and compensation that is required for piezo-resistive, ceramic capacitive diaphragm and silicon diaphragm technologies. Furthermore, these types are not cost effective at current technology levels.
The use of optical sensing is not practical due to the possibility of contaminated oil.
A Hall effect sensor was ruled out because ferrous contamination would cause problems with the magnet.
The best solution to the sensing problem appeared to be some form of electromagnetic sensor, and from this premise the present invention evolved and has been developed to its current form. The invention provides a solution to a difficult control problem and satisfies the above constraints. A durable, highly reliable sensor is provided in a very small package for direct mounting to the pump body and insertion directly into and across the high and low pressure sides of the pump. Internal damping is provided within the small sensor itself to damp high pressure pulses and other rapid pressure variations, without the need for coiled copper tubing and the like. Furthermore, the sensor includes an internal reference for compensating for various surrounding parameters such as temperature.